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Hearing

2 Structure and function

2.1 Structure and function of the outer and middle ear

Figure 1 is a diagram of the human ear. The outer ear consists of the visible part of the ear or pinna, the external auditory canal (meatus), and the tympanic membrane (tympanum) or eardrum. The human pinna is formed primarily of cartilage and is attached to the head by muscles and ligaments. The deep central portion of the pinna is called the concha, which leads into the external auditory canal, which in turn leads to the tympanic membrane.

Figure 1 The human ear showing the outer, middle and inner ears

Only mammals have pinnae and only some have mobile pinnae. The pinnae of humans and primates have no useful muscles and are therefore relatively immobile. Mobile, and to some extent, immobile pinnae help in localising sounds by funnelling them towards the external canal.

Activity

How would the immobility of our pinnae affect how we localise a sound source?

Answer

Unlike animals with mobile pinnae, we must reposition our head in order to aim our ears at a sound source.

The pinnae also help in distinguishing between noises originating in front of and behind the head, and in providing other types of filtering of the incoming sound wave. In addition, the concha and external auditory canal effectively enhance the intensity of sound that reaches the tympanic membrane by about 10 to 15 dB. This enhancement is most pronounced for sounds in the frequency range of roughly 2 to 7 kHz and so, in part, determines the frequencies to which the ear is most sensitive. Finally, the outer ear protects the tympanic membrane against foreign bodies and changes in humidity and temperature.

The external auditory canal extends about 2.5 cm inside the skull before it ends in the tympanic membrane. Sound travels down the meatus and causes the tympanic membrane to vibrate. The tympanic membrane is thin and pliable so that a sound, consisting of compressions and rarefactions of air particles, pulls and pushes at the membrane moving it inwards and outwards at the same frequency as the incoming sound wave. It is this vibration that ultimately leads to the perception of sound. The greater the amplitude of the sound waves, the greater the deflection of the membrane. The higher the frequency of the sound, the faster the membrane vibrates.

On the other side of the tympanic membrane is the middle ear (Figure 1) which is an air-filled chamber containing three interlocking bones called ossicles. These are the smallest bones in the body and function to transmit the vibrations caused by auditory stimulation at the tympanic membrane to the inner ear. The bones are called the malleus (Latin for ‘hammer’), the incus (‘anvil’) and the stapes (‘stirrup’). The ossicle attached to the tympanic membrane is the malleus, which forms a rigid connection with the incus. The incus forms a flexible connection with the stapes. The flat bottom portion of the stapes, the footplate, is connected to the oval window (a second membrane covering a hole in the bone of the skull). In response to sound, the inward-outward movement of the tympanum displaces the malleus and incus and the action of these two bones alternately drives the stapes deeper into the oval window and retracts it, resulting in a cyclical movement of fluid within the inner ear.

This may seem a complex way to transmit vibrations of the tympanic membrane to the oval window. Why must they be transmitted via the ossicular chain and not simply transferred directly?

The reason is that the middle ear cavity is air-filled while the inner ear is fluid-filled. The passage of sound information from the outer to the inner ear involves a boundary between air and fluid. If you have tried talking to someone who is under water, you may have observed that sound does not travel efficiently across this kind of interface. In fact, 99.9 per cent of the sound energy incident on an air/fluid boundary is reflected back within the air medium and only 0.1 per cent is transmitted to the fluid. Therefore, if sound waves were to impinge directly on the oval window, the membrane would barely move. Most of the sound would be reflected back because the fluid in the inner ear is denser than air and resists being moved much more than air does. Consequently, in order to drive the movement of the oval window and vibrate the fluid, greater pressure is needed.

The middle ear provides two ways of doing this. The first is to do with the relative sizes of the tympanic membrane and the stapes footplate (which is connected to the oval window). Measurements have shown that the area of the tympanic membrane that vibrates in response to high intensity sound is 55 mm2. The stapes footplate which makes contact with the oval window has an area of only about 3.2 mm2. So, if all the force exerted on the tympanic membrane is transferred to the stapes footplate, then the pressure (force per course area) must be greater at the footplate because it is smaller than the tympanic membrane. One rather painful demonstration of this principle is to compare the pressure exerted on your toe by someone wearing a stiletto heel compared to the pressure exerted by the same person wearing an ordinary trainer.

The second way in which the middle ear ossicles transfer the force from the tympanic membrane to the stapes footplate is through the lever action of the ossicles. Figure 2 shows how a lever system can increase the force of an incoming signal.

Figure 2 The lever action of the middle ear. The length of the malleus corresponds to D1, the distance between the applied force and the fulcrum. The length of the incus corresponds to D2, the distance between the fulcrum and the resultant force. If D2 is less than D1, then the resultant force will be greater than the applied force

The middle ear has another function in addition to the mechanical transformation of the auditory signal. When the auditory system is subjected to very loud sounds that are potentially harmful to the inner ear, two set of muscles, the tensor tympani and the stapedius muscles, contract and in so doing reduce the magnitude of the vibration transmitted through the middle ear. The response of these muscles to loud noises is known as the acoustic or middle ear reflex.

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